High-pressure High-temperature Treatment Research Articles

High Pressure Synthesis, Structure, Physical and Mechanical Properties of C60 Fullerite Superhard and Ultrahard Phases.

New data on the synthesis conditions, structure and physical properties of the superhard and ultrahard samples created from solid C60 by static high-pressure high-temperature treatment in the range of 9. 5-13 GPa ; 600 -2100 K are represented. Measured velocities of the compressional sound waves in the ultrahard fullerites exceed the highest values for diamond. Bulk elastic modulus for the hardest fullerite samples more than twice exceeds that of diamond. Ultrahard fullerite tip is applied for measurements of hardness of diamond and fullerites. Maximum hardness of fullerite is measured to be 310±40 GPa.

Electronic structure of tungsten and molybdenum germanides synthesized at high pressures

X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS) methods were used to study the electronic structure of WGe2, W5Ge3, and MoGe2 compounds formed in a high temperature-high pressure treatment. Transformations of the electronic structure after termination of the α → β irreversible polymorphic transitions in the tungsten and molybdenum digermanides have been investigated. The XES bands and XAS edges in all compounds studied have been derived and compared in a single energy scale. The XPS core-level binding energies have been determined. In the germanides a small amount of charge transfers from the Ge atoms to the M atoms (M - W, Mo). In the molybdenum and tungsten germanides the M d(p)-like valence states and Ge 4p-like states are strongly hybridized.

The kinetics of solidification of Al-Si eutectic alloys under high pressure

Abstract The kinetics of crystallization of eutectic alloys Al100-xSix (χ=12, 18, 26 at.%) was investigated under pressures p=0.5, 2.5, 4.5 GPa. The values of supercooling and average grain size of silicon crystals were determined for alloys quenched from melt under different pressures with the cooling rate 103 K/s. The data obtained were used to evaluate quantitatively the pressure dependences of surface tension (between melt and crystal) and activation energy of crystal growth which, in turn, have made it possible to determine the relative change of nucleation frequency and of the rate of crystal growth with pressure. The possibility is shown, based on the investigation of mechanical properties of the samples obtained under high pressure, for improving the strength and the plasticity of A1-Si alloys by means of high pressure-high temperature treatment.

Mössbauer study of small amounts of iron in graphite, around the diamond-graphite pressure-temperature stability region

An exploratory Mossbauer spectroscopy study of the Fe-C system in the C rich region, prepared by high pressure-high temperature treatment near the graphite-diamond stability line, was made. The results obtained for the different processing conditions give no evidence of Fe intercalation in graphite. The presence of some water in the cell produced hydrated Fe complexes, which can explain the deleterious effect of water or hydrogen in the high pressure diamond synthesis.

Hydrostatic Pressure Effect on Oxygen Precipitates in Silicon Single Crystal

Czochralski-grown Si single crystals (Cz-Si) subjected to hydrostatic pressure (HP) can change defect stucture and so the bulk properties influenced by defects. This effect is related mostly to the oxygen-induced defects existing in the bulk of annealed samples [1-4], e.g. oxygen precipitates (OP) composed of SiO2 [3]. In this paper we investigate the effect of HP on the defect structure of Cz-Si monocrystals containing OP. The isostatic and pulsating HP as well as ultrasound (US) treatments at room temperature were applied. Also the high pressure-high temperature (HP-HT) treatment was used to modify the elastic misfit e at the OP/Si boundary [3]. The OP density dop in the preliminary annealed Cz-Si single crystals (oxygen concentration cO up to 10 18 cm-3) was in the range of (10 5-107) cm-2 . The samples were subjected to the HP treatment up to 1.8 GPa, to the HP-HT treatment at HT up to 1620 K or to the HP-US treament at HP up to 1 GPa (the US amplitude up to 9 MPa, frequency 17 kHZ) [5]. The HP induced changes of

Formation of molybdenum nitrides by ammonia nitridation of MoCl 5

The formation of molybdenum nitride from MoCl 5 and flowing NH 3 was investigated at temperatures of 770–1060 K. Up to 910 K the reaction yields single-phase WC-type δ-MoN with lattice parameters of a = 0.2851(2) nm and c = 0.2782(3) nm. At T ≥ 950 K, a reaction product consisting of the two phases δ-MoN + γ-MoN 1− x was obtained, where the amount of γ-phase increased with increasing reaction temperature. The γ-MoN 1− x lattice parameter is dependent on the reaction temperature, whereas the γ-MoN lattice parameters remain unchanged. At reaction temperatures of 1060 K, a reaction product with a conspicuosly different, coarse-grained facetted habit was formed - grown probably by a transport mechanism via the gas phase - with sharp diffraction lines and lattice parameters of a = 0.5740(2) nm and c = 0.5624(3) nm, which are significantly larger than the doubled cell parameters of the WC-type MoN. Debye-Schrrer photographs of this nitride suggest that it is isostructural with the recently observed MoN resulting from high-pressure high-temperature treatment of WC-type δ-MoN.

INFLUENCE OF LOW TEMPERATURE HIGH PRESSURE AND HIGH TEMPERATURE HIGH PRESSURE TREATMENT ON STRUCTURE AND SUPERCONDUCTIVITY OF AMORPHOUS ALLOY La80Al20

The rapidly quenched La80Al20 alloy is amorphous. At atmospheric pressure, its crystallization temperature and glass temperature are ～280℃ and ～242℃ respec-tivily, according to DTA measurements. The superconducting transition temperature Tc increases from 3.87 to 4.18K, in the hydrostatic pressure range of 1-4770 bars, while its residual resistance decreases significantly. After high pressure-high temperature treatment (25kbar, ～160℃, 40 min), amorphous→crystalline transition of La80Al20 alloy occurs, as disclosed by X-ray diffraction method. The product consists of α-La phase and a new metastable phase, and its Tc increases from 3.9 to 5.7 K. Some preliminary discussion on the experimental results are also presented.

Transformation of three-connected silicon nets in CaSi 2

Up to 40 kbar and 1100°C, CaSi 2 is dimorphic. Trigonal/rhombohedral CaSi 2I (CaSi 2-type structure) with corrugated layers of three-connected Si atoms can be transformed by a high pressure-high temperature treatment into tetragonal CaSi 2II (α-ThSi 2-type structure) with a three-dimensional net of three-connected Si atoms. The silicon net of CaSi 2II is slightly distorted from the topologically simplest tetragonal three-dimensional three-connected net derived on a geometrical basis. In order to correlate crystal chemical with thermochemical data the transformation between both polymorphs of CaSi 2 has been studied at equilibrium and nonequilibrium conditions. The pressure-temperature phase diagram of CaSi 2 has been investigated by X-ray technique in quenched samples. From the slope of the equilibrium line and the change in molar volume the approximate values of the entropy and energy of transformation CaSi 2(I-II) have been determined ΔS = 3.2 e.u., ΔU = 4.9 kcal/mole. Under nonequilibrium conditions the transformation CaSi 2(II-I) yielded ΔH = −4.2 kcal/mole at 500°C and ambient pressure in a DTA apparatus. Complete transformation of metastable CaSi 2II can be achieved within 5 min at a heating rate of 20°C/min. Due to the relatively high speed of transformation simple structural relations between both polymorphs of CaSi 2 are discussed.

Notizen: Umwandlung gewellter Si-Schichten in einen Si-Raumnetzverband / Transformation of Corrugated Si-Layers into a Three-Dimensional Si-Network

Abstract Corrugated Si-layers in trigonal Ca0,65Sr0,35Si2 (EuGe2 type of structure, D3d 3 -P 3̄m l, a = 389.1 pm, c = 504.8 pm, Z = 1, zSi = 0,420) are transformed by a high pressure-high temperature treatment into a tetragonal three-dimensional Si-network (a = ThSi2 type of structure, D4h 19-I 41/amd, a = 433.6 pm, c = 1367 pm, Z = 4, zSi = 0.414).

Semiconductor-metal transition in BaSi 2

Orthorhombic BaSi 2 (BaSi 2-type structure) is a diamagnetic semiconductor at ambient pressure. By a high pressure-high temperature treatment in a belt-type apparatus a trigonal polymorph (EuGe 2-type structure) is obtained which is quenchable. The trigonal polymorph is a diamagnetic semimetal. The semiconductor-metal transition in barium disilicide has been studied in the reverse direction (metallic trigonal to semiconducting orthorhombic polymorph), since heating the trigonal phase to 400°C at ambient pressure results in a complete transformation into the orthorhombic phase. The characterization of the phase transformation was performed by investigating the changes in molar volume, enthalpy, electrical resistance and magnetic susceptibility. These changes are discussed from a crystal chemical point of view.

Pressure composition effects on superconducting A15 compounds

The phase stability of "bulk" A15 compounds prepared using ordinary metallurgical technique is contrasted with high pressure-high temperature synthesis in relation to the formation of high T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> "stoichiometric" compounds. Alloys of Nb-Ge, Nb-Al, and Nb-Ga were prepared by standard metallurgical procedures and subsequently submitted to simultaneous high pressure-high temperature treatment. These materials were characterized as to phase development and superconducting properties to evaluate the influence of the high pressure treatment. The results indicate that high T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> materials may ultimately be obtained by high pressure-high temperature synthesis. The high pressure phase stability of A15 compounds is discussed. The phase formation of off-stoichiometric high T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> A15 compounds is also presented in terms of a non-equilibrium process involving a high quench rate.